Conductive paste composition and semiconductor devices made therewith

ABSTRACT

The present invention provides a thick-film paste composition for printing the front side of a solar cell device having one or more insulating layers. The thick-film paste comprises an electrically conductive metal and an oxide composition dispersed in an organic medium that includes an organopolysiloxane and a fluorine-containing degradation agent.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 62/410,969, filed Oct. 21, 2016, and entitled “Conductive PasteComposition and Semiconductor Devices Made Therewith,” which applicationis incorporated herein in its entirety for all purposes by referencethereto.

FIELD OF THE INVENTION

The present disclosure relates to a conductive paste composition that isuseful in the construction of a variety of electrical and electronicdevices, and more particularly to a paste composition useful in creatingconductive structures, including electrodes for photovoltaic devices,devices constructed with such paste compositions, and a process forconstructing these devices.

TECHNICAL BACKGROUND

Photovoltaic (PV) devices (also called solar cells) are now widely usedto convert incident sunlight into usable electrical energy, which cansupply an external load of any desired type. The cells are provided withelectrodes that are configured to be connected to wires through whichthe generated electricity flows to the load. Of course, it is desiredthat the generation and transmission of energy be done as efficiently aspossible, to maximize the amount of the incident solar energy that canbe captured and turned into usable electrical energy. Technologists havedevoted attention in recent years to many ways of increasing thatefficiency. Solar-powered photovoltaic systems are considered to beenvironmentally beneficial in that they reduce the need for older formsof generation, such as burning fossil fuels in conventional electricpower plants.

Conventional solar cells are based on a structure that includes ajunction between semiconducting materials with differentmajority-carrier conductivity types, such as a p-n junction formedbetween an n-type semiconductor and a p-type semiconductor. Morespecifically, crystalline Si photovoltaic cells are typically made byadding controlled impurities (called dopants) to purified silicon, whichis an intrinsic semiconductor. Dopants from IUPAC group 13 (e.g., B) aretermed “acceptor dopants” and produce p-type material, in which themajority charge carriers are positive “holes,” or electron vacancies.Dopants from IUPAC group 15 (e.g., P) are termed “donor dopants” andproduce n-type material, in which the majority charge carriers arenegative electrons. Dopants may be added to bulk materials by directinclusion in the melt during silicon crystal growth. Doping of a surfaceis often accomplished by providing the dopant at the surface in eitherliquid or gaseous form, and then thermally treating the basesemiconductor to cause the dopants to diffuse inward. Ion implantation,possibly with further heat treatment, is also used for surface doping.

The cell structure includes a boundary or junction between p-type andn-type Si. When the cell is illuminated by electromagnetic radiation ofan appropriate wavelength, such as sunlight, a potential (voltage)difference develops across the junction as the electron-hole pair chargecarriers migrate into the electric field region of the p-n junction andbecome separated. The spatially separated charge carriers are collectedby electrodes in contact with the surfaces of the semiconductor. Thecell is thus adapted to supply electric current to the connectedelectrical load. Since sunlight is almost always the light source,photovoltaic cells are commonly known as “solar cells.”

In the commonly used planar p-base configuration, a negative electrodeis located on the side of the cell that is to be exposed to a lightsource (the “front,” “light-receiving,” or “sun” side; a positiveelectrode is located on the other side of the cell (the “back” or“non-illuminated” side). Cells having a planar n-base configuration, inwhich the p- and n-type regions are interchanged from the p-baseconfiguration, are also known.

Both electrodes are normally provided by suitable metallizations, i.e.,thin layers of electrically conductive metal situated on some or all ofone or both surfaces of the cell. Most commonly, these electrodes areprovided on opposite sides of a generally planar cell structure.Conventionally, they are produced by applying suitable conductive metalpastes to the respective surfaces of the semiconductor body andthereafter firing the pastes. For example, U.S. Pat. No. 8,497,420discloses a method of manufacturing a solar cell electrode comprisingsteps of: applying onto a semiconductor substrate a conductive pastecomprising (i) a conductive powder such as Ag, (ii) alead-tellurium-oxide based glass frit, (iii) ethyl cellulose as anorganic polymer, (iv) suitable thixotropes and surfactants; and (v) asolvent comprising predominantly 2,2,4-trimethyl-1,3-pentanediolmonoisobutyrate; and firing the conductive paste to produce an electrodesuitable for devices such as solar cells.

Most photovoltaic cells are fabricated with an insulating layer on theirfront side to afford an anti-reflective property that maximizes theutilization of incident light. However, in this configuration, theinsulating layer normally must be removed to allow an overlaidfront-side electrode metallization to make contact with the underlyingsemiconductor surface. Conductive metal pastes appointed for fabricatingfront side electrodes typically include a glass frit and a conductivespecies (e.g., silver particles) carried in an organic medium thatfunctions as a vehicle for printing. The electrode may be formed bydepositing the paste composition in a suitable pattern (for instance, byscreen printing) and thereafter firing the paste composition andsubstrate.

The specific formulation of the paste composition has a strong buthighly unpredictable effect on both the electrical and mechanicalproperties of electrodes constructed therewith. To obtain goodelectrical characteristics for the finished cell (e.g., high lightconversion efficiency and low resistance), the composition mustpenetrate or etch fully through the anti-reflective layer during firingso that a good electrical contact is established, but without damagingthe underlying semiconductor.

Ideally, the electrode has high electrical conductivity and alow-resistance connection to the underlying device to minimize the lossof efficiency from ohmic heating within the cell. In addition, it isdesirable for the total area of the electrode to be as small as possibleto avoid the loss of efficiency that results from shadowing of theincident light on the light-receiving surface. Ordinarily, theserequirements necessitate a structure that includes plural fineconductive lines. The conductivity of the lines is improved byincreasing their cross-sectional area in the plane perpendicular totheir length. But to minimize shadowing, the fired line should be highbut narrow. However, with existing paste compositions, it has provendifficult both to form such lines by screen printing and to preventexcessive line spreading during firing. It is further desired that astrongly adhering bond between the electrode and the substrate uponfiring is formed. Still further, it is desirable that the vehicle iscompletely removed during firing, so that there is no residue thatdegrades the conductivity of the electrode. With many conventional pastecompositions, it thus has not proven possible to reliably fire theprinted wafers so that good adhesion and electrical properties areobtained in combination.

Although various methods and compositions useful in forming devices suchas photovoltaic cells are known, there nevertheless remains a need forcompositions that permit fabrication of patterned conductive structuresthat provide improved overall device electrical performance and thatfacilitate the rapid and efficient manufacture of such devices in bothconventional and novel architectures.

SUMMARY

An aspect of the present disclosure provides a paste composition,comprising: an inorganic solids portion that comprises:

(a) a source of electrically conductive metal, and

(b) an inorganic oxide-based component, and a vehicle in which theconstituents of the inorganic solids portion are dispersed, the vehiclecomprising:

(c) an organopolysiloxane;

(d) a fluorine-containing degradation agent; and

(e) a solvent.

In certain embodiments, the fluorine-containing degradation agentcomprises a substance comprising monovalent cations, such as one havingthe formula M⁺X⁻, wherein M⁺ is a monovalent cation and X⁻ is F⁻ or(HF₂)⁻, or a substance comprising cations having a valence of 2+ ormore, such as one having the formula [M^(k+)][X⁻]_(k), wherein M^(k+) isa cation with positive charge k and X⁻ is F⁻ or (HF₂)⁻.

Another aspect provides a process comprising:

-   -   (a) providing a semiconductor substrate having opposing first        and second major surfaces and comprising an insulating layer        situated on the first major surface of the semiconductor        substrate;    -   (b) applying a paste composition as recited herein onto at least        a portion of the first major surface, and    -   (c) firing the semiconductor substrate and the paste composition        under conditions sufficient to form an electrode in electrical        contact with the semiconductor substrate.

Also disclosed are articles that are formed using the present pastecomposition in the practice of the foregoing processes. Such articlesinclude semiconductor devices and photovoltaic cells. The presentprocesses can be used to form electrodes contacting siliconsemiconductors, the electrodes comprising electrically conductivestructures formed by any of the processes described herein.

A further aspect of the disclosure provides a photovoltaic cellprecursor, comprising:

-   -   (a) a semiconductor substrate having opposing first and second        major surfaces; and    -   (b) a paste composition as recited herein, the paste composition        being applied onto a preselected portion of the first major        surface and configured to be formed by a firing operation into        an electrically conductive structure comprising an electrode in        electrical contact with the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is made to the following detaileddescription of the preferred embodiments of the invention and theaccompanying drawings, wherein like reference numerals denote similarelements throughout the several views and in which:

FIGS. 1A to 1F are drawings in cross-section view for explaining a solarcell electrode manufacturing process; and

FIG. 2 is a plot of thermogravimetric data taken on two neat PDMSmoieties and a mixture of PDMS and TBAF.

DETAILED DESCRIPTION

Various aspects of the present disclosure relate to the need for highperformance semiconductor, photovoltaic, and other electronic deviceshaving mechanically robust and durable, high conductivity electrodes aswell as processes suitable for their manufacture.

One aspect provides a paste composition that comprises as functionalinorganic materials a conductive component, such as a source ofelectrically conductive metal, and an oxide-based component such as aglass frit that are dispersed in a predominantly organic vehicle thatcomprises an organopolysiloxane and a degradation agent, along withsolvents and other optional functional agents such as polymers,surfactants, thickeners, thixotropes, and binders. In another aspect,the paste composition is beneficially employed in the fabrication ofhigh-quality electrodes of photovoltaic devices. Ideally, the pastecomposition promotes the formation of a metallization that: (a) providesa relatively low resistance contact with the substrate; (b) preservesthe electrical characteristics of the underlying substrate; and (c)adheres strongly to the underlying semiconductor substrate. A PV cellemploying such a metallization as one or both of its electrodes ideallyexhibits good electrical and mechanical characteristics, including oneor more of high photovoltaic conversion efficiency, high fill factor,low series resistance, high shunt resistance, and good mechanicaladhesion between the electrode and the substrate. Suitable pastecompositions are believed to aid in etching surface insulating layers,which are ordinarily included in semiconductor structures such asphotovoltaic cells, as required for making good contact between theconductive electrode and the underlying semiconductor.

As further described below, the vehicle of the paste composition acts asa carrier for the inorganic solids constituents, which are dispersedtherein. Along with solvent, the vehicle may include one or morecomponents such as polymers, surfactants, thickeners, thixotropes, andbinders that may impart desirable functional properties, includingwithout limitation ones desirable in deposition and electrode formationprocesses. The vehicle is predominantly composed of organic materials,but small amounts of inorganic materials that enhance the functionalityof the vehicle are optionally included.

Typically, electrodes or other like conductive traces are provided byscreen printing the paste composition onto a substrate, although otherforms of deposition may alternatively be used, including, withoutlimitation, plating, extrusion or co-extrusion, dispensing from asyringe, inkjet, shaped, multiple, or ribbon printing. After deposition,the composition is fired at an elevated temperature. A separate dryingstep is optionally carried out prior to the actual firing.

The present composition also can be used to form conductive traces forother purposes, such as those employed in a semiconductor module that isto be incorporated into an electrical or electronic device. As would berecognized by a skilled artisan, the paste composition described hereincan be termed “conductive,” meaning that the composition can be formedinto a structure and thereafter processed to exhibit an electricalconductivity sufficient for conducting electrical current betweendevices and circuitry connected thereto.

I. Inorganic Components A. Electrically Conductive Metal

The present paste composition includes a source of an electricallyconductive metal. Exemplary metals include without limitation silver,gold, copper, nickel, palladium, platinum, aluminum, and alloys andmixtures thereof. In some embodiments, the electrically conductive metalis selected from the group consisting of Ag, Cu, and Pd; alternatively,the electrically conductive metal consists essentially of silver, whichis beneficial for its processability and high conductivity. However, acomposition including at least some non-precious metal may be used toreduce cost or to modify other properties.

The conductive metal may be incorporated directly in the present pastecomposition as a metal powder. In another embodiment, a mixture of twoor more such metals or an alloy is directly incorporated. Alternatively,the metal is supplied by a metal oxide or salt that decomposes uponexposure to the heat of firing to form the metal. As used herein, theterm “silver” is to be understood as referring to elemental silvermetal, alloys of silver, and mixtures thereof, and may further includesilver derived from silver oxide (Ag₂O or AgO) or silver salts such asAgCl, AgNO₃, AgOOCCH₃ (silver acetate), AgOOCF₃ (silvertrifluoroacetate), Ag₃PO₄ (silver orthophosphate), or mixtures thereof.Any other form of conductive metal compatible with the other componentsof the paste composition also may be used in certain embodiments. Othermetals used in the present paste for the functional conductive materialmay be similarly derived.

Electrically conductive metal powder used in the present pastecomposition may be supplied as finely divided particles having anymorphology, including without limitation, any one or more of thefollowing morphologies: a powder form, a flake form, a spherical form, arod form, a granular form, a nodular form, a layered or coated form,other irregular forms, or mixtures thereof. The electrically conductivemetal or source thereof may also be provided in a colloidal suspension,in which case the colloidal carrier would not be included in anycalculation of weight percentages of the solids of which the colloidalmaterial is part.

The particle size of the metal is not subject to any particularlimitation. As used herein, “particle size” is intended to refer to“median particle size” or d₅₀, by which is meant the 50% volumedistribution size. The particle size distribution may also becharacterized by d₉₀, meaning that 90% by volume of the particles aresmaller than d₉₀. Volume distribution size may be determined by a numberof methods understood by one of skill in the art, including but notlimited to laser diffraction and dispersion methods employed by aMicrotrac particle size analyzer (Montgomeryville, Pa.). Laser lightscattering, e.g., using a model LA-910 particle size analyzer availablecommercially from Horiba Instruments Inc. (Irvine, Calif.), may also beused. In various embodiments, the median size of the metal particlesranges between 0.01, 0.2, 0.3, or 0.5 μm and 10 μm; or between 0.3, 0.4,or 0.5 μm and 5 or 8 μm, as measured using the Horiba LA-910 orMicrotrak analyzers. Particles having diameters in these ranges arefound to sinter well. Furthermore, the particle diameter should becommensurate with the functioning of the method used to apply theconductive paste onto a semiconductor substrate, for example, screenprinting. In an embodiment, it is possible to mix two or more types ofconductive powder of different diameters and/or morphologies.

In an embodiment, the conductive powder is of ordinary high purity(99%). However, depending on the electrical requirements of theelectrode pattern, conductive powders with higher or lower purity canalso be used.

As further described below, the electrically conductive metal or asource thereof is dispersed in a vehicle that acts as a carrier for themetal phase and other constituents present in the formulation. Theelectrically conductive metal may comprise any of a variety ofpercentages of the composition of the paste composition. To attain highconductivity in a finished conductive structure, it is generallypreferable for the concentration of the electrically conductive metal tobe as high as possible while maintaining other required characteristicsof the paste composition that relate to either processing or final use,such as the need for a uniform, mechanically robust and adherent contactand adequate penetration of any surface passivation and/oranti-reflective coating present on the substrate. Minimizing the bulkresistivity and the contact resistance between the conductive structureand the underlying device beneficially tends to decrease the sourceresistance of a device.

In an embodiment, the conductive powder comprises 50 weight percent (wt.%) or more of the total weight of the conductive paste. In otherembodiments, the conductive powder comprises an amount of the conductivepaste between a lower limit of 50, 60, 70, 75, 80, or 85 wt. % and anupper limit of 80, 82.5, 85, 87.5, 90, 92.5, or 95 wt. %. In otherembodiments, the silver or other electrically conductive metal maycomprise about 75% to about 99.75% by weight, or about 85% to about99.5% by weight, or about 95% to about 99% by weight, of the inorganicsolids component of the paste composition. In another embodiment, thesolids portion of the paste composition may include about 80 wt. % toabout 90 wt. % silver particles and about 1 wt. % to about 9 wt. %silver flakes. In an embodiment, the solids portion of the pastecomposition may include about 70 wt. % to about 90 wt. % silverparticles and about 1 wt. % to about 9 wt. % silver flakes. In anotherembodiment, the solids portion of the paste composition may includeabout 70 wt. % to about 90 wt. % silver flakes and about 1 wt. % toabout 9 wt. % of colloidal silver. In a further embodiment, the solidsportion of the paste composition may include about 60 wt. % to about 90wt. % of silver particles or silver flakes and about 0.1 wt. % to about20 wt. % of colloidal silver.

The electrically conductive metal used herein, particularly when inpowder form, may be coated or uncoated; for example, it may be at leastpartially coated with a surfactant to facilitate processing. Suitablecoating surfactants include, for example, stearic acid, palm itic acid,a salt of stearate, a salt of palmitate, and mixtures thereof. Othersurfactants that also may be utilized include lauric acid, oleic acid,capric acid, myristic acid, linoleic acid, and mixtures thereof. Stillother surfactants that also may be utilized include polyethylene oxide,polyethylene glycol, benzotriazole, poly(ethylene glycol)acetic acid,and other similar organic molecules. Suitable counter-ions for use in acoating surfactant include without limitation hydrogen, ammonium,sodium, potassium, and mixtures thereof. When the electricallyconductive metal is silver, it may be coated, for example, with aphosphorus-containing compound. It is understood that the pastecomposition may separately include one or more surfactants, apart fromany surfactant present as a coating of conductive metal powder.

B. Oxide component

The present paste composition includes a fusible inorganic oxidematerial. The term “fusible,” as used herein, refers to the ability of amaterial to become fluid upon heating, such as the heating employed in afiring operation. In some embodiments, the fusible material is composedof one or more fusible subcomponents. For example, the fusible materialmay comprise a glass material, or a mixture of two or more glassmaterials that may have different softening and/or meltingcharacteristics. Glass material in the form of a fine powder, e.g., asthe result of a comminution operation, is often termed “frit” and isbeneficially employed as the oxide material of some embodiments of thepresent paste composition.

While the present invention is not limited by any particular theory ofoperation, it is believed that in some embodiments, the glass frit (orother like oxide material) and the frit additive (if present) act inconcert during firing to efficiently penetrate the insulating layernormally present on the wafer, such as a naturally occurring orintentionally formed passivation layer and/or an antireflective coating.Such a result is frequently termed “firing through.” The glass frit andfrit additive are also thought to promote sintering of the conductivemetal powder, e.g. silver, that forms the electrode in some embodiments.

As used herein, the term “glass” refers to a particulate solid form,such as an oxide or oxyfluoride, that is at least predominantlyamorphous, meaning that short-range atomic order is preserved in theimmediate vicinity of any selected atom, that is, in the firstcoordination shell, but dissipates at greater atomic-level distances(i.e., there is no long-range periodic order). Hence, the X-raydiffraction pattern of a fully amorphous material exhibits broad,diffuse peaks, and not the well-defined, narrow peaks of a crystallinematerial. In the latter, the regular spacing of characteristiccrystallographic planes give rise to the narrow peaks, whose position inreciprocal space is in accordance with Bragg's law.

Upon initial heating, glass materials undergo certain structural changestypically denominated as the glass transition. In general, and withoutbeing bound by any theory, it is understood that there is a transitionfrom a low temperature state in which the constituent atoms are tightlybound, to a semi-viscous state, in which thermal energy permits theatoms to become more mobile. The glass transition is manifested inchanges that can be seen in measurements of a variety of physicalphenomena, including without limitation calorimetric and mechanicalmeasurements.

In accordance with typical usage in the art of glass chemistry, the term“glass transition temperature,” or “T_(g),” is used herein to refer tothe onset temperature for this transition as measured calorimetrically.As described in ASTM Standard Test Method E-1356-08, T_(g) isconveniently determined empirically using conventional DSC or DTAmeasurements, as the temperature of intersection of two tangents drawnto the calorimetric curve, one in the baseline region below thetransition region and one at the steepest portion of the curve in thetransition region. DSC and DTA data are frequently collected at aconstant heating rate of 10° C./min. (ASTM Standard Test Methods arepromulgated by ASTM International, West Conshohocken, Pa. Each such ASTMstandard referenced herein is incorporated in its entirety for allpurposes by reference thereto.)

The softening point of a fusible material herein is understood torepresent the temperature (T_(s)) above which the logarithm of thematerial's viscosity η (measured in Pas) drops below 6.6, in accordancewith conventional usage, e.g. as set forth in “Materials Letters,” Vol.31, p 99-103 (1997) and ASTM Standard Test Method C1351M-96.

In an embodiment, the softening point of glass material used in thepresent paste composition is in the range of 300 to 800° C. In otherembodiments, the softening point is in the range of 250 to 650° C., or300 to 500° C., or 300 to 400° C., or 390 to 600° C., or 400 to 550° C.,or 410 to 460° C. Glass frits having such softening points can meltproperly to obtain effects such as those mentioned above. Alternatively,the “softening point” can be obtained by the fiber elongation method ofASTM C338-93.

It is also contemplated that some or all of the fusible oxide materialmay be composed of material that exhibits some degree of crystallinity.For example, in some embodiments, a plurality of oxides are meltedtogether, resulting in a material that is partially amorphous andpartially crystalline. As would be recognized by a skilled person, sucha material would produce an X-ray diffraction pattern having narrow,crystalline peaks superimposed on a pattern with broad, diffuse peaks.Alternatively, one or more constituents, or even substantially all ofthe fusible material, may be predominantly or even substantially fullycrystalline. In certain embodiments, crystalline material useful in thefusible material of the present paste composition may have a meltingpoint of at most 700° C., 750° C., or 800° C.

Ordinarily, the particle size of the oxide material is not critical,provided the paste can be uniformly prepared and deposited. In certainembodiments, the median particle size of the oxide material can rangefrom a lower limit of 0.1, 0.3, 0.4, 0.5, 0.6, or 0.8 μm to an upperlimit of 1, 3, 5, 7, or 10 μm. With such particle size, the oxidematerial can be uniformly dispersed in the paste. The particle size(d₅₀) can be obtained in the same manner as described above for theconductive powder.

The chemical composition of the oxide material or glass frit of thepresent paste composition is not limited. Any glass frit suitable foruse in electrically conducting pastes for electronic materials isacceptable. For example, and without limitation, lead borosilicate, leadsilicate, and lead tellurium glass frits can be used. For example, leadtellurium oxide-containing glass frits useful in the present pastecomposition include without limitation ones provided by U.S. Pat. Nos.8,497,420, 8,895,843, and 8,889,979, which are all incorporated hereinfor all purposes by reference thereto. In an embodiment, the glass fritcomprises a lead tellurium oxide, such as one comprising PbO and TeO₂,with a mole ratio of lead to tellurium of the oxide being between 5/95and 95/5. In other embodiments, the glass frit comprises a leadtellurium boron oxide or a lead tellurium lithium oxide. Any of theseglass frits may comprise any of: 10 to 75, 25 to 60, or 30 to 50 wt % ofPbO; 10 to 70, 25 to 65, or 40 to 60 wt % TeO₂; 0.1 to 15, 0.25 to 5, or0.4 to 2 wt % B₂O₃; or 0.1 to 7.5, 0.2 to 5, 0.2 to 3, or 0.3 to 1 wt. %Li₂O. In addition, zinc borosilicate or other lead-containing orlead-free glasses can be also used.

In various embodiments, the glass frit can be 0.25 to 12 wt. %, 0.25 to8 wt. %, 0.5 to 6 wt. %, 0.5 to 4 wt. %, or 1.0 to 3 wt. % based on thetotal weight of the conductive paste.

The embodiments of the glass frit or like material described herein arenot limiting. It is contemplated that one of ordinary skill in the artof glass chemistry could make minor substitutions of additionalingredients and not substantially change the desired properties of thegiven composition, including its interaction with a substrate and anyinsulating layer thereon.

The oxide component of the present paste composition is understood torefer to a composition containing anions of one or more types, of whichat least 80% are oxygen anions, and cations. In various embodiments, atleast 90%, 95%, 98%, or substantially all the anions of the oxidecomponent are oxygen anions.

In some embodiments, the oxide component comprises a mixture of finelydivided powders of at least two separate fusible materials that havedistinct chemical compositions. Each of the fusible materialsindependently may be either crystalline or partially or fully glassy oramorphous. In most embodiments, at least the first fusible material is aglass frit material. The at least two fusible materials have differentsoftening and/or melting characteristics. In an embodiment, thedifferent behavior operates to enhance the electrical and mechanicalcharacteristics obtained after firing the solar cell precursor.

A median particle size of the fusible materials in the presentcomposition may be in the range of about 0.5 to 10 μm, or about 0.8 to 5μm, or about 1 to 3 μm, as measured using the Horiba LA-910 analyzer.

In an embodiment, the thick film paste may include the oxide compositionin an amount of 0.25 to 15%, 0.25 to 8%, 0.5 to 5%, or 1 to 3%, byweight based on solids.

One of ordinary skill in the art of glass chemistry would recognize thatthe fusible materials herein are commonly described as includingpercentages of certain components. Specifically, the composition of thefusible materials can be specified by denominating individual componentsthat may be combined in the specified percentages to form a startingmaterial that subsequently is processed, e.g., as described herein, toform a glass or other fusible material. Such nomenclature isconventional to one of skill in the art. In other words, the fusiblematerials contain certain components, and the percentages of thosecomponents may be expressed as weight percentages of the correspondingoxide or other forms.

Alternatively, the composition of the fusible material herein may beexpressed in cation percentages, which are based on the total cationscontained in the particular material, unless otherwise indicated by thecontext. Of course, compositions thus specified include the oxygen orother anions associated with the various cations in the amounts requiredfor charge balance. A skilled person would recognize that compositionscould equivalently be specified by weight percentages of theconstituents, and would be able to perform the required numericalconversions.

A skilled person would further recognize that any of the fusiblematerials herein, whether specified by weight percentages, molarpercentages, or cation percentages, e.g. of the constituent oxides, mayalternatively be prepared by supplying the required anions and cationsin requisite amounts from different components that, when mixed andheated, yield the same overall composition. For example, in variousembodiments, lithium for the compound Li₂O could be supplied either fromthe oxide directly or alternatively from a suitable organic or inorganiclithium-containing compound (such as Li₂CO₃) that decomposes on heatingto yield Li₂O. The skilled person would also recognize that a certainportion of volatile species, e.g., carbon dioxide, may be releasedduring the process of making a fusible material.

It is known to those skilled in the art that the cations of some of theoxides described herein exist in more than one stable valence oroxidation state. For example, cobalt is known in multiple possibleoxidation states, with cobalt(II), cobalt(III), and cobalt(II,III)oxides, respectively having formulas CoO, Co₂O₃, and Co₃O₄, beingreported. Fusible materials herein that include such cations can beprepared using any of the known oxides, or compounds that form oxidesupon heating in air.

A skilled person would also recognize that a fusible material such asone prepared by a melting technique as described herein may becharacterized by known analytical methods that include, but are notlimited to: Inductively Coupled Plasma-Emission Spectroscopy (ICP-ES),Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), andthe like. In addition, the following exemplary techniques may be used:X-Ray Fluorescence spectroscopy (XRF), Nuclear Magnetic Resonancespectroscopy (NMR), Electron Paramagnetic Resonance spectroscopy (EPR),Mössbauer spectroscopy, electron microprobe Energy DispersiveSpectroscopy (EDS), electron microprobe Wavelength DispersiveSpectroscopy (WDS), and Cathodoluminescence (CL). A skilled person couldcalculate percentages of starting components that could be processed toyield a particular fusible material, based on results obtained with suchanalytical methods.

One of ordinary skill in the art would recognize that the choice of rawmaterials could unintentionally include impurities that may beincorporated into the glass during processing. For example, theimpurities may be present in the range of hundreds to thousands ppm. Thepresence of the impurities would not alter the properties of the glass,the thick film composition, or the fired device. For example, a solarcell containing the thick-film composition may have the efficiencydescribed herein, even if the thick-film composition includesimpurities.

Preparation of Fusible Materials

In an embodiment, the fusible materials comprised in the present oxidecomposition may be produced using any suitable technique and equipment,including those conventionally employed in the glass-making arts. Forexample, the ingredients may be weighed and mixed in the requisiteproportions and then heated in a platinum alloy crucible in a furnace ata temperature sufficient to melt the constituents together. Theingredients may be heated to a peak temperature (e.g., 800° C. to 1400°C., or 1000° C. to 1200° C., or 900° C. to 1050° C.) and held for a timesuch that the material forms a melt that is substantially liquid andhomogeneous (e.g., 20 minutes to 2 hours). The melt optionally isstirred, either intermittently or continuously. In an embodiment, themelting process results in a material wherein the constituent chemicalelements are homogeneously and intimately mixed at an atomic level. Themolten material is then typically quenched in any suitable wayincluding, without limitation, passing it between counter-rotatingstainless steel rollers to form 0.25 to 0.50 mm thick platelets, bypouring it onto a thick stainless steel plate, or by pouring it into asuitable quench fluid. The resulting particles are then milled to form apowder or frit, which typically may have a d₅₀ of 0.2 to 3.0 μm (e.g. asmeasured using a Horiba LA-910 analyzer). For example, the milling maybe carried out in a polyethylene container with zirconia media andisopropyl alcohol or water optionally containing 0.5 wt. % TRITON™ X-100octylphenol ethoxylate surfactant (available from Dow Chemical Company,Midland, Mich.). The comminuted powder may be recovered bycentrifugation or filtration and then dried.

Other production techniques may also be used for the present fusiblematerials and other oxide-based materials. One skilled in the art ofproducing such materials might therefore employ alternative synthesistechniques including, but not limited to, melting in non-precious metalcrucibles, melting in ceramic crucibles, sol-gel, spray pyrolysis, orothers appropriate for making powder forms of glass.

Any size-reduction method known to those skilled in the art can beemployed to reduce particle size of the constituents of the presentpaste composition to a desired level. Such processes include, withoutlimitation, ball milling, media milling, jet milling, vibratory milling,and the like, with or without a solvent present. If a solvent is used,water is the preferred solvent, but other solvents may be employed aswell, such as alcohols, ketones, and aromatics. Surfactants may be addedto the solvent to aid in the dispersion of the particles, if desired.

C. Optional Oxide Additive

The inorganic oxide material in the present paste composition mayoptionally comprise a plurality of separate fusible substances, such asone or more frits, or frit with another crystalline frit additivematerial, or small amounts of other known inorganic additives. Withoutlimitation, one such additive that has been found useful is a lithiumruthenium oxide, as set forth in U.S. Pat. No. 8,808,581 to VerNooy etal., which is incorporated herein by reference thereto for all purposes.In various embodiments, the frit additive may comprise 0.01-2%,0.05-1.5%, or 0.1-1%, based on the total weight of the conductive paste.

II. Vehicle

The inorganic components of the present composition are typically mixedwith a vehicle to form a relatively viscous material referred to as a“paste” or an “ink” that has a consistency and rheology that render itsuitable for printing processes, including without limitation screenprinting. Without being bound by any theory of operation, it is believedthat the vehicle plays an important role in determining the rheology ofthe paste composition, and thus the ability to print it as fine linesthat remain high and narrow after firing, so that shading is minimizedand conductivity is increased. The inclusion of an organopolysiloxaneand a fluorine-containing degradation agent in combination is believedto be beneficial for improving these characteristics.

The present paste composition is typically formulated with a mechanicalmixing system, and the constituents may be combined in any order, aslong as they are uniformly dispersed and the final formulation hascharacteristics such that it can be successfully applied during end use.Mixing methods that provide high shear are especially useful for someformulations.

The vehicle typically provides a medium in which the inorganiccomponents are dispersible with a good degree of stability of thechemical and functional properties of the paste composition. Inparticular, the paste composition preferably has a stability compatiblenot only with the requisite manufacturing, shipping, and storage, butalso with conditions encountered during deposition, e.g., by a screenprinting process. Ideally, the rheological properties of the vehicle aresuch that it lends good application properties to the paste composition,including stable and uniform dispersion of solids, appropriate viscosityand thixotropy for printing, appropriate wettability of the paste solidsand the substrate on which printing will occur, a rapid drying rateafter deposition, and stable firing properties. As defined herein, thevehicle is not considered to be part of the inorganic solids comprisedin the thick-film paste composition.

Ideally, a firing operation removes the materials contained in thevehicle without leaving any residue that causes effects detrimental tothe electrical or mechanical properties of the finished conductivestructure or the substrate.

The proportions of vehicle and inorganic solids components in thepresent paste composition can vary in accordance with the method ofapplying the paste and the specific composition of the vehicle. In anembodiment, the present paste composition typically contains about 50 to95 wt. %, 76 to 95 wt. %, or 85 to 95 wt. %, of the inorganic componentsand about 5 to 50 wt. %, 5 to 24 wt. %, or 5 to 15 wt. %, of thevehicle, and substances associated therewith.

A. Organopolysiloxane Lubricating Agent

The vehicle of the present conductive paste composition includes atleast one organopolysiloxane. Without being bound by any theory, it isbelieved that the organopolysiloxane acts as a lubricating agent thatfacilitates the passage of the paste through a screen during printing offine lines or other defined features of a conductive structure. The term“organopolysiloxane” refers generally to a mixed inorganic-organicpolymer that includes an Si—O—Si backbone, as depicted by the structureof Formula (I):

wherein each R^(J) is independently at each occurrence an organic sidegroup. Each of the R^(J) groups may be as simple as a lower alkyl oraryl group (e.g., a methyl, ethyl, or phenyl group) or may be an organicgroup of another type or a larger group, any of which may optionallyinclude substituents; and n is an integer greater than 1. A variety ofsuch organopolysiloxanes are available commercially, e.g. from Gelest,Inc, Morrisville, Pa., and Dow Corning Corporation, Auburn Mich.

Suitable organopolysiloxanes include, without limitation,polyalkylsiloxanes such as polydimethylsiloxane, polydiethylsiloxane,and polyoctylmethylsiloxane; and polyarylsiloxanes such as polydiphenylsiloxane and polymethyl phenyl siloxane. In an embodiment, the siloxaneshave the general structure depicted by Formula (II),

wherein each of the R¹, R², and R³ groups is independently selected fromC1-C8 alkyl, C2-C8 alkenyl, C2-C8 alkynyl, or C6-C10 aryl groups; n isan integer ranging from 50 to 100,000, inclusive. Any one, two, or threeof the R¹, R², and R³ groups may be identical, and any of them mayoptionally comprise one or more substituents selected from alkoxy,hydroxy, carbonyl, carboxyl, amino, epoxy, methacrylic, glycidoxy,ureido, sulfide, methacryloxy, sulfhydryl, and halogen (F, Cl, Br)groups.

In an embodiment, the R¹, R², and R³ groups are all the same, forexample all being methyl groups. In other embodiments, R¹ and R² are thesame, such as methyl groups, and R³ is another functional group asdelineated above, such as a hydroxy group. In still other embodiments R¹is a group of one type, e.g. a methyl group, and both R² and R³ areother, different groups, one or both optionally being a functional groupas defined above.

Suitable lubricating agents also include organosiloxane copolymers,including random or block copolymers, with a chemical structure depictedby Formula (III),

wherein each of R¹, R², R³ and R⁴ groups is independently selected fromC1-C8 alkyl, C2-C8 alkenyl, C2-C8 alkynyl, or C6-C10 aryl group, any ofwhich optionally may comprise one or more substituents selected fromalkoxy, hydroxy, carbonyl, carboxyl, amino, epoxy, methacrylic,glycidoxy, ureido, sulfide, methacryloxy, sulfhydryl, and halogen (F,Cl, Br) groups; and m and n are integers, each of which may rangeindependently from 50 to 100,000, inclusive. The R¹ and R⁴ groups aredifferent from each other. For example, and without limitation, thepaste composition may containalkylmethylsiloxane-arylalkylmethylsiloxane copolymers such asethylmethylsiloxane-co-2-phenylpropylmethylsiloxane copolymer.

In other embodiments of the present paste composition, theorganopolysiloxane includes a mixture in any proportion of more than oneof the polymers or copolymers described above.

In an embodiment, the organopolysiloxane used in the present pastecomposition forms a phase-segregated mixture when mixed with the rest ofvehicle carrier as described herein. The expression “phase-segregatedmixture” refers herein to a liquid mixture in which, at roomtemperature, are present visible liquid domains having an averagediameter greater than 0.1 μm, even after vigorous mixing or heating,such as provided by extended centrifugal mixing and heating at 60° C.for 1 h. Suitable mixing can be carried out in a THINKY® mixer(available from Thinky® USA, Inc., Laguna Hills, Calif.) at 2000 rpm for30 sec. Such domains can be imaged using optical microscopy in eithertransmission or reflection mode.

In an embodiment, the present paste composition includes anorganopolysiloxane, such as a trimethyl- or hydroxy-terminated PDMS,that has a number average molecular weight (Mn) ranging from a lowerlimit of 5, 7.5, 10, 50, 75, or 100 kilodaltons (kDa) to an upper limitof 120, 150, 200, 250, 500, or 1,000 kDa. Ordinarily,organopolysiloxanes with too low a molecular weight are partially orfully dissolved in the paste composition's solvent and so do not exhibitadequate phase separation for the desired improvement in printingrheology to be achieved. Organopolysiloxanes with too high a molecularweight are themselves typically viscous, limiting the amount that can beadded without rendering the final paste composition too viscous to bereadily printed, so that no end-use benefit is realized. Suitablemolecular weights also depend on the particular terminal group(s)present. In some embodiments, functionalized termination of theorganopolysiloxane results in a lower paste viscosity and betterprintability than would be seen at the same loading with anun-functionalized organopolysiloxane having the same molecular weight.This, in turn, permits the loading of the organopolysiloxane and/or itsmolecular weight to be increased in a paste that exhibits desirablerheology. Without limitation, the organopolysiloxane may have akinematic viscosity ranging from about 50 or 100 to about 100,000,200,000, or 500,000 centistokes (cSt).

Embodiments of the present paste composition include ones in which theamount of organopolysiloxane ranges from a lower limit of 0.05%, 0.1%,0.2%, or 0.3% to an upper limit of 0.75%, 1%, or 2% by weight of thetotal paste composition.

B. Degradation Agent

It has been found that the presence of an organopolysiloxane inconductive paste compositions facilitates the printing of fine lines orother like features in a conductive structure. However, the gains inefficiency and other PV cell electrical properties expected fromprinting fine lines are generally not fully realized. It is believedthat the organopolysiloxane is not completely removed during the firingoperation, especially for higher molecular weight forms of theorganopolysiloxane.

The present inventors find that the further inclusion of a thermallyactivated fluoride-containing degradation agent in the paste compositionpermits the fine-line printing benefits to be better realized incombination with improved final cell electrical properties. Withoutbeing bound by any theory of operation, it is believed that thedegradation agent permits more complete removal of theorganopolysiloxane during the firing operation, and after the conductivestructure has been printed and dried.

Thus, the organic vehicle of the present paste composition furtherincludes at least one degradation agent for the organopolysiloxane. Asused herein, the expression “degradation agent” refers to any substancethat promotes thermal degradation of the organopolysiloxane at a lowertemperature than for the organopolysiloxane itself. The enhancement ofdegradation can be identified using thermogravimetric analysis (TGA) tocompare the behavior of the neat organopolysiloxane with that of amixture of the organopolysiloxane and the degradation agent. Thedegradation is manifest in weight loss that occurs at a lowertemperature for the mixture than for the neat organopolysiloxane.

The degradation of the organopolysiloxane may be characterized by T₅₀,i.e. the temperature at which 50% of the weight change seen in TGA at aconstant 10° C./min heating rate in air has occurred. In someembodiments, the ratio of the amounts of degradation agent andorganopolysiloxane present in the paste composition is such that T₅₀ forthe mixture is reduced by at least 100, 200, 250, or 300° C. below thatof the neat organopolysiloxane.

In an embodiment, the degradation agent contains fluorine, which isbelieved to cause depolymerization by cleavage of bonds in the Si—O—Sibackbone of the organopolysiloxane.

In an embodiment, the degradation agent may comprise a substancecomprising monovalent cations, such as a substance having the generalformula M⁺X⁻. In an embodiment, X⁻ is F⁻ or (HF₂)⁻ and M⁺ is amonovalent cation. In one such embodiment, M⁺ has the structure given bythe structure of Formula (IV),

or R¹R²R³R⁴N^(⊕), wherein each of R¹ to R⁴ is independently H or anyalkyl or aryl group having at least 1 or 3 and up to 4, 5, 7, 15, 20,30, or 100 carbons. Without limitation, any of the aryl groups may be acyclic, polycyclic, or heterocyclic aromatic hydrocarbon and any of thealkyl groups may linear or branched or a cyclic, polycyclic, orheterocyclic hydrocarbon and may be a singly or multiply substitutedhaloalkyl (Cl or Br), hydroxyalkyl, thioalkyl, or ether group. Exemplarymaterials useful as fluorine-containing degradation agents in thepresent paste composition include tetrabutylammonium fluoride (TBAF),tetramethylammonium fluoride (NMe4F), and trimethylbenzylammoniumfluoride.

In another such embodiment, the degradation agent is an inorganicfluoride salt, such as a salt having the formula M⁺X⁻, wherein M⁺ is amonovalent metal cation, including without limitation Ag⁺ and the alkalimetal cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺), or a mixture thereof. Thedegradation agent may also be an ammonium-fluoride salt including,without limitation, ammonium fluoride (NH₄F), ammonium hydrogendifluoride (NH₄HF₂), hydrazonium fluoride (NH₂NH₃F). or hydrazoniumhydrogen difluoride (NH₂NH₃HF₂). Alternatively, fluoride substancescomprising cations that are higher than monovalent may be used, such asa salt or other substance having a formula [M^(k+)][X⁻]_(k), whereinM^(k+) is a cation with positive charge k and X⁻ is F⁻ or (HF₂)⁻.

In still another embodiment the degradation agent is hexafluorosilicicacid (H₂SiF₆) or a hexafluorosilicate, including, without limitation,ammonium hexafluorosilicate ((NH₄)₂SiF₆) or sodium hexafluorosilicate(Na₂SiF₆).

FIG. 2 depicts the effect of a representative fluoride-containingdegradation agent (TBAF) on methyl-terminated and hydroxy-terminatedforms of PDMS (both with a MW of about 110 kDa). Thermogravimetric (TGA)traces, obtained in a TA Q500 analyzer (available from TA Instruments,New Castle, Del.) operated with a 10° C./min heating rate in air, areshown for the two PDMS moieties and for a mixture of methyl-terminatedPDMS with 7.3% by weight of TBAF. It may be seen that substantial weightloss (indicating degradation) for both neat PDMS materials occurred overapproximately 500 to 600° C. and 400 to 500° C. for themethyl-terminated and hydroxy-terminated forms of PDMS, traces 100 and110, respectively. In contrast, a mixture of 7.3 wt. % of TBAF withmethyl-terminated PDMS resulted in a degradation that was complete byabout 150° C., trace 120. The TGA data for hydroxy-terminated PDMS with7.3% TBAF was virtually identical to trace 120, demonstrating that theTBAF strongly promotes thermal degradation of PDMS in both forms, withthe T₅₀ value being decreased by about 390° C. and 290° C.,respectively.

The present paste composition may include any amount of degradationagent that is effective to enhance thermal degradation of anorganopolysiloxane contained in a conductive paste. In an embodiment,the amount of degradation agent ranges from a lower limit of 0.001%,0.005%, or 0.01% to an upper limit of 0.1%, 0.3%, 0.6%, or 1%, based onthe total paste composition. Too much degradation agent will thin thepaste composition undesirably in some instances. In other embodiments,the amount is such that a ratio of the number of F atoms present in thedegradation agent to the number of Si atoms present in theorganopolysiloxane ranges from a lower limit of 0.0002, 0.001, 0.002,0.005, or 0.01 to an upper limit of 0.05, 0.1, or 0.2.

C. Other Polymeric Materials

In addition to the foregoing organopolysiloxane polymer(s), the presentpaste composition may include one or more additional polymericmaterials. Such materials include, without limitation, any one or moreof the substances disclosed in U.S. Pat. No. 7,494,607 and InternationalPatent Application Publication No. WO 2010/123967 A2, both of which areincorporated herein in their entirety for all purposes, by referencethereto. These include ethyl- and ethylhydroxyethyl-based cellulosicpolymers, cellulose acetate, cellulose acetate butyrate, wood rosin andderivatives thereof, mixtures of ethyl cellulose and phenolic resins,polymethacrylates of lower alcohols, and monobutyl ether of ethyleneglycol monoacetate. For example, ethylcellulose, as provided by DowChemical Company, Midland, Mich. in varying viscosities under thetradenames Ethocel™ STD 4, STD 10, and STD 200, may be used. Thesematerials are said by their manufacturer to have an ethoxyl content of48.0 to 49.5% and to act as rheology modifiers and binders. Alsopossible are Vamac® G diamine-cured terpolymer of ethylene,methylacrylate, and a cure site monomer elastomer (E. I. DuPont deNemours and Company, Wilmington Del.); and Foralyn™ 110 pentaerythritolester of hydrogenated rosin (Eastman Chemical, Kingsport, Tenn.). Any ofthe polymers above or other suitable polymers may be present in thevehicle in any effective amount.

In some possible embodiments, the organic polymer (beyond theorganopolysiloxane and exclusive of solvent) can be 0.01% or 0.05% to 1,2, 3, or 5.0% by weight of the paste composition, as long as a viscositythat permits deposition by screen printing or the like is maintained.

D. Other Vehicle Constituents

The vehicle may further comprise one or more other non-aqueous organicsubstances including, without limitation, surfactants, wetting agents,dispersants, thickeners, thixotropes, other rheology- orviscosity-adjusting agents, stabilizers, binders, or other commonadditives known to those skilled in the art. The organic vehicle mayalso include naturally-derived ingredients such as various plant-derivedoils, saps, resins, or gums. The additional organic substancesordinarily are non-aqueous and are inert, meaning that they may beremoved by a firing operation without leaving any substantial residue orproducing other effects detrimental to the paste or the final conductorline properties.

Surfactants found useful in the present paste composition include,without limitation: Duomeen® TDO surfactant (Akzo Nobel SurfaceChemistry, LLC, Chicago, Ill.); Tween® surfactant (Aldrich), apolyoxyethylene sorbitol ester represented by the manufacturer as havinga calculated molecular weight of 1,225 Da, assuming ethylene oxideunits, 1 sorbitol, and 1 lauric acid as the primary fatty acid; andsodium dodecyl sulfate (SDS). Suitable wetting agents include phosphateesters and soya lecithin.

A wide variety of inorganic and organic thixotropic agents are useful,including gels, organics, and agents derived from natural sources suchas castor oil (possibly hydrogenated) or a derivative thereof. Suchsubstances promote shear thinning behavior in some embodiments.Thixatrol® MAX and Thixatrol® PLUS amides (Elementis Specialties, Inc.,Hightstown, N.J.) are exemplary thixotropic rheology modifiers. Otherlow molecular weight amides or amide-olefin oligomers may also besuitable. It is, of course, not always necessary to incorporate athixotropic agent since the solvent and resin properties coupled withthe shear thinning inherent in any suspension may alone be suitable inthis regard.

E. Solvent

The vehicle of the present paste composition ordinarily includes one ormore solvents in which the other organic and inorganic substances of thecomposition are dispersed. The proportion of solvent is frequentlyadjusted at the end of production or immediately prior to use, so thatthe paste composition has a viscosity compatible with the desiredprinting or other method of application. Further beneficial effects ofthe solvent(s) include any one or more of: dissolving any organic resinscontained in the paste; and stabilizing a concentrated suspension of theinorganic solids present. Ideally the solvent and other organics can becompletely removed during a firing operation. Some of the solvent may besufficiently volatile to promote rapid hardening after the pastecomposition is applied on a substrate.

Solvents known for use in paste compositions include ester alcohols suchas Texanol™ solvent (2,2,4-trimethyl-1,3-pentanediol monoisobutyrate);terpenes such as alpha- or beta-terpineol or mixtures thereof with othersolvents such as kerosene, dibutylphthalate, butyl carbitol, butylcarbitol acetate (diethylene glycol n-butyl ether acetate), hexyleneglycol, dibenzyl ether; benzyl alcohol or other higher alcohols andalcohol esters, benzyl benzoate, 2-pyrrolidone, dibasic ester (DBE), orany mixture thereof. Some embodiments incorporate volatile liquids forpromoting rapid hardening after application on the substrate can beincluded in the vehicle. Various combinations of these and othersolvents are formulated to obtain the viscosity and volatilityrequirements desired, along with other beneficial paste properties. Thepresent paste composition may be adjusted as needed to a predetermined,screen-printable viscosity, e.g., by adding additional solvent(s).

In an embodiment, the vehicle may include one or more componentsselected from the group consisting of: bis(2-(2-butoxyethoxy)ethyl)adipate, dibasic esters, octyl epoxy tallate, and isotetradecanol. Thepaste composition may also include additional additives or components.

The dibasic ester useful in the present paste composition may compriseone or more dimethyl esters selected from the group consisting ofdimethyl ester of adipic acid, dimethyl ester of glutaric acid, anddimethyl ester of succinic acid. Various forms of such materialscontaining different proportions of the dimethyl esters are availableunder the DBE® trade name from Invista (Wilmington, Del.). For thepresent paste composition, a preferred version is sold as DBE-3 and issaid by the manufacturer to contain 85 to 95 weight percent dimethyladipate, 5 to 15 weight percent dimethyl glutarate, and 0 to 1.0 weightpercent dimethyl succinate based on total weight of dibasic ester.

The solvent can be 2 to 10 wt. % in an embodiment, 4 to 9 wt. % inanother embodiment, and 5 to 8 wt. % in another embodiment, based on theweight of the conductive paste. With such amount of solvent, aconductive paste can exhibit a viscosity and other rheologicalproperties suitable for deposition, e.g. by screen printing.

The conductive paste composition may have any viscosity that iscompatible with the desired deposition process. A skilled person willrecognize that the viscosity required for best printability depends on anumber of factors, including, without limitation, the screen mesh usedand the size of the finger lines and other structures to be printed.Frequently, the paste composition is adjusted prior to deposition byaddition of a small amount of the solvent. In some implementations, afinal viscosity at 25° C. of about 100±50 Pas or more at 50 rpm has beenfound convenient for screen printing fine electrode lines. In otherembodiments, the viscosity at 25° C. is 80 to 100±20 Pa·s or 100 to110±20 Pas after 3 min at 50 rpm. The viscosity of the conductive pastecan be measured with Brookfield HBT viscometer with a utility cup usinga #14 spindle or other similar apparatus, with values being taken after3 min at 50 rpm.

In some embodiments, one or more of the components of the vehiclepromotes thixotropy, or shear thinning. The degree of shear thinning canbe ascertained from a difference seen between viscosity measurementscarried out with different rotation times and rates, e.g., by comparingvalues obtained at 0.5 rpm (3 min), 10 rpm (3 min), 20 rpm (3 min)and/or 50 rpm (3 min).

III. Formation of Conductive Structures A. Substrate

An aspect of the present disclosure provides a process that may be usedto form a conductive structure on a substrate. Ordinarily, the processfirst entails the fabrication of a precursor structure, generallycomprising the steps of providing the substrate and applying a pastecomposition onto it in a preselected pattern suitable for producing theconductive structure in the desired final configuration. The precursoris then fired to produce the conductive structure, which is often termeda “metallization.” Most commonly, the substrate is planar and relativelythin, thus defining opposing first and second major surfaces on itsrespective sides. The present paste composition may be used to form anelectrode on one or both of these major surfaces.

Substrates appointed for manufacturing photovoltaic cells normallyinclude an emitter region formed by doping the front side with a dopantof the type needed to produce the desired majority-carrier conductivitytype. This doping typically involves exposing the wafer to an elevatedtemperature in a thermal cycle designed to achieve the desired dopingprofile of carrier concentration versus depth. Some such cycles involveheating in ambient atmosphere, which may result in some amount ofsurface oxidation of the silicon substrate material.

B. Insulating layer

In some embodiments, the present paste composition is used inconjunction with a substrate, such as a semiconductor substrate, havingan insulating or passivation layer present on one or both of thesubstrate's major surfaces. The layer may comprise, without limitation,one or more components selected from aluminum oxide; titanium oxide;silicon nitride; SiN_(x):H (silicon nitride containing hydrogen forpassivation during subsequent firing processing); silicon oxide; siliconnitride, oxide, or oxynitride containing carbon; and siliconoxide/titanium oxide. There may be a single, homogeneous layer ormultiple sequential sub-layers, each of which independently may be anyof these materials. Silicon nitride and SiNx:H are widely used.Insulating layers between 1 and 200 nm thick are suitable for typicalapplications.

In implementations for fabricating photovoltaic cells, the insulatinglayer is typically structured to provide an anti-reflective property, tolower the amount of incident light that is reflected from the cell'ssurface. Reducing the amount of light lost to reflection improves thecell's utilization of the incident light and increases the electricalcurrent it can generate. Thus, the insulating layer is often denoted asan anti-reflective coating (ARC). The configuration of the layer(whether a single layer of one material or a plurality of separatelyfabricated sublayers that may be different in composition) preferably ischosen to maximize the anti-reflective property in accordance with thelayer material's composition and refractive index. For example, theinsulating ARC layer may have a thickness of between 1 and 200 nm. Inone approach, the deposition processing conditions are adjusted to varythe stoichiometry of the layer, thereby altering properties such as therefractive index to a desired value. For a single silicon nitride layerwith a refractive index of about 1.9 to 2.0, a thickness of about 700 to900 Å (70 to 90 nm) is suitable, but other choices may also be used.

The insulating layer may be deposited on the substrate by methods knownin the microelectronics art, such as any form of chemical vapordeposition (CVD) including plasma-enhanced CVD (PECVD) and thermal CVD,thermal oxidation, or sputtering. In another embodiment, the substrateis coated with a liquid material that under thermal treatment decomposesor reacts with the substrate to form the insulating layer. In stillanother embodiment, the substrate is thermally treated in the presenceof an oxygen- or nitrogen-containing atmosphere to form an insulatinglayer. Alternatively, no insulating layer is specifically applied to thesubstrate, but a naturally forming substance, such as silicon oxide on asilicon wafer, may function as an insulating layer.

The present method optionally includes the step of forming theinsulating layer on the semiconductor substrate prior to the applicationof the paste composition.

In some implementations of the present process, the paste composition isuseful whether the insulating layer or any constituent sublayer thereofis specifically applied or naturally occurring. The paste's oxide andnon-oxide components may act in concert to combine with, dissolve, orotherwise penetrate some or all of the thickness of any insulating layermaterial during firing.

C. Application

The present composition can be applied as a paste onto a preselectedportion of a major surface of a semiconductor substrate in a variety ofdifferent configurations or patterns, depending on the devicearchitecture and the particular substrate material used. The preselectedportion may comprise any fraction of the total area of the majorsurface. The area covered may range from a small fraction up tosubstantially all of the area. In an embodiment, the paste is applied ona semiconductor substrate, which may be single-crystal, cast mono,multi-crystal, polycrystalline, or ribbon silicon, or any othersemiconductor material.

The application can be accomplished using a variety of depositionprocesses, including screen printing and other exemplary depositionprocesses discussed above. In an embodiment, the paste composition maybe applied over any insulating layer present on the pertinent majorsurface of the substrate.

The conductive composition may be printed in any useful pattern. Forexample, the application of the conductive paste may be used to form aphotovoltaic cell precursor, wherein the paste is deposited on apreselected portion of a semiconductor substrate in a configuration thatis appointed to be converted by a firing operation into an electricallyconductive structure that includes at least one electrode in electricalcontact with the substrate. In an implementation, the at least oneelectrode is configured to be connected to outside electrical circuitryto which electrical energy is to be supplied.

The electrode pattern used for a front side electrode of a photovoltaiccell commonly includes a plurality of narrow grid lines or fingersextending from one or more larger bus bars. Such a pattern permits thecurrent generated in the cell to be extracted from the front sidewithout undue resistive loss, while minimizing the area obscured by themetallization, which inherently reduces the amount of incoming lightenergy that can be converted to electrical energy. Ideally, the featuresof the electrode pattern should be well defined, with preselected anduniform thickness and shape, and provide high electrical conductivityand low contact resistance with the underlying structure. Fingers thatare uniform in height and width and have a high ratio of height to widthare beneficial in increasing the effective conductor cross sectionalarea (thus decreasing electrical resistance) while minimizing theobscured area. Thus, a paste composition ideally has rheology such thatfinger lines can be uniformly deposited in the initial printing withoutundulations or other irregularities in width or height and that furtherspreading or other distortion does not occur during subsequent drying orfiring steps.

In an embodiment, the width of the lines of the conductive fingers maybe 20 to 200 μm; 25 to 100 μm; or 25 to 50 μm. In an embodiment, thethickness of the lines of the conductive fingers may be 5 to 50 μm; 10to 35 μm; or 10 to 25 μm.

D. Firing

A heat treatment operation often termed “firing” is used in the presentprocess to promote the formation of a conductive structure that includesan electrode providing a high-quality electrical contact with anunderlying substrate, such as a semiconductor wafer in a PV(photovoltaic) cell. A drying operation optionally precedes the firingoperation to harden the paste composition, which may comprise removingits most volatile organics. It may be carried out through either anextended exposure to a relatively modest elevated temperature (e.g., atemperature of up to about 150 to 175° C.) or a shorter durationexposure to a higher temperature. In an embodiment, the conditions ofthe firing operation (e.g., the temperature attained by the precursorand duration of exposure) are sufficient to form an electrode exhibitingthe desired properties in its electrical contact with the associatedsemiconductor substrate.

The firing operation is believed to effect a substantially completeburnout of the vehicle from the deposited paste by volatilization and/orpyrolysis of the constituent vehicle materials. While the presentinvention is not limited by any particular theory of operation, it isbelieved that during a suitable firing, the fusible material acts toefficiently penetrate the insulating layer normally present on thewafer, such as a naturally-occurring or intentionally formed passivationlayer and/or an anti-reflective coating. Such a result is frequentlytermed “firing through.” The various paste components are also thoughtto promote sintering of the conductive metal powder, e.g. silver, thatforms the electrode.

Ideally, the firing results in formation of an electrode that has goodelectrical properties, including a high bulk conductivity and a lowsurface resistivity connection to the underlying semiconductor material,thereby reducing the source impedance of the cell. While someembodiments may function with electrical contact that is limited toconductive domains dispersed within the printed area, it is preferredthat the contact be uniform over substantially the entire printed area.It is also beneficial for the conductive metal structure to bemechanically robust and securely attached to the substrate, with ametallurgical bond being formed over substantially all the area of thesubstrate covered by the conductive element.

Such a paste would further enable screen-printed crystalline siliconsolar cells to have reduced saturation current density at the frontsurface (J0e) and accompanying increased Voc and Jsc, and thereforeimproved solar cell performance. Other desirable characteristics of apaste would include high bulk conductivity and the ability to formnarrow, high-aspect-ratio contact lines in a metallization pattern tofurther reduce series resistance and minimize shading of incident lightby the electrodes, as well as good adherence to the substrate. A highshunt resistance is also desired, indicating that the firing did notadversely affect the semiconductor's properties.

In one embodiment, the set point temperature of the oven or furnace forthe firing may be in the range between about 300° C. and about 1000° C.,or between about 300° C. and about 525° C., or between about 300° C. andabout 650° C., or between about 650° C. and about 950° C. The firing maybe conducted using any suitable heat source, and may be performed in anatmosphere composed of air, nitrogen, an inert gas, or anoxygen-containing mixture such as a mixed gas of oxygen and nitrogen.

In an embodiment, the firing is accomplished using a belt furnace. Thesubstrate bearing the printed paste composition pattern is placed on abelt that is conveyed through the furnace's hot zone at high transportrates, for example between about 100 to about 500 cm per minute, withresulting hold-up times between about 0.05 to about 5 minutes. Multipletemperature zones may be used to control the desired thermal profile inthe furnace, and the number of zones may vary, for example, between 3 to11 zones. The temperature of a firing operation conducted using a beltfurnace is conventionally specified by the furnace set point in thehottest zone of the furnace, but it is known that the highesttemperature actually attained by the passing substrate in such a processis somewhat lower than the highest set point. Other batch and continuousrapid fire furnace designs known to one of skill in the art are alsocontemplated.

E. Semiconductor Device Manufacture

An embodiment of the present disclosure relates to a device structurecomprising a substrate and a conductive electrode, which may be formedby the process described above.

Conductive structures as provided herein may be usefully employed in awide range of electrical, electronic, and semiconductor devices. Withoutlimitation, such devices include photodiodes, photovoltaic cells, andsolar panels or other like articles, in which one or more conductivestructures function as electrodes through which the device can beconnected to other electrical circuitry. Devices that are individuallyor collectively fabricated using processes disclosed herein may beincorporated into larger structures, such as a solar panel including aplurality of interconnected photovoltaic cells. Ordinarily, precursorsformed using the process disclosed herein or any other suitable processare converted into finished semiconductor devices using a firingoperation that converts the deposited paste composition into a suitablyconfigured conductive structure that provides electrodes in electricalcommunication with the semiconductor, as described herein.

One possible sequence of steps implementing the present process formanufacture of a photovoltaic cell device is depicted by FIGS. 1A-1F.While the process is described with reference to a conventional p-basecell having a planar architecture, comparable steps useful infabricating planar n-base cells or cells having other architectures suchas interdigitated back contact cells will also be apparent.

FIG. 1A shows a p-type substrate 10, which may be any known type of Siincluding, without limitation, single-crystal, multi-crystalline,mono-crystalline, or polycrystalline silicon. For example, substrate 10may be obtained by slicing a thin wafer from an ingot that has beenformed from a pulling or casting process. In an implementation, the Siingot is doped with B to render it p-type. Surface damage andcontamination (from slicing with a wire saw, for example) may be removedby etching away about 10 to 20 μm of the substrate surface using anaqueous alkali solution such as aqueous potassium hydroxide or aqueoussodium hydroxide, or using a mixture of hydrofluoric acid and nitricacid. In addition, the substrate may be washed with a mixture ofhydrochloric acid and optional hydrogen peroxide to remove heavy metalssuch as iron adhering to the substrate surface. Although notspecifically depicted, substrate 10 may have a first major surface 12that is textured to reduce light reflection. Texturing may be producedby etching a major surface with an aqueous alkali solution such asaqueous potassium hydroxide or aqueous sodium hydroxide. Substrate 10may also be formed from a silicon ribbon.

In FIG. 1B, an n-type diffusion layer 20 is formed on the first majorsurface 12 to create a p-n junction with p-type material below. Then-type diffusion layer 20 can be formed by any suitable doping process,such as thermal diffusion of phosphorus (P) provided from phosphorusoxychloride (POCl₃) or ion implantation. The profile of dopantconcentration versus depth from the surface depends on the amount ofmaterial deposited and the implantation and/or thermal conditions used.The particular thermal treatment applied may also affect how muchsilicon oxide, if any, is formed on the wafer surface. As shown, then-type diffusion layer 20 is formed over the entire surface of thesilicon p-type substrate. In other implementations, the diffusion layeris confined to the top major surface, obviating the need for the removalprocess. The depth of the diffusion layer can be varied by controllingthe diffusion temperature and time, and is generally formed in athickness range of about 0.3 to 0.5 μm. The n-type diffusion layer mayhave a sheet resistivity ranging from several tens of ohms per square upto about 120 ohms per square. In some alternative implementations (notshown), additional doping with B at a level above that of the bulk isadded in a layer on opposing second (rear) major surface 14.

After protecting one surface of the n-type diffusion layer 20 with aresist or the like, the n-type diffusion layer 20 is removed from mostsurfaces by etching so that it remains only on the first major surface12 of substrate 10, as shown in FIG. 1C. The resist is then removedusing an organic solvent or the like.

Next, as shown in FIG. 1D, an insulating layer 30, which also functionsas an anti-reflective coating, is formed on the n-type diffusion layer20. The insulating layer is commonly silicon nitride, but can also be alayer of another material, such as SiNx:H (i.e., the insulating layercomprises hydrogen for passivation during subsequent firing processing),titanium oxide, silicon oxide, mixed silicon oxide/titanium oxide,aluminum oxide, or another suitable insulating material. The insulatinglayer can be in the form of a single layer or multiple sublayers of thesame or different materials. In some embodiments, the production of thediffusion layer shown in FIG. 1B includes a thermal treatment in airthat may result in the formation of some amount of silicon dioxide onthe substrate surface (not shown).

Next, electrodes are formed on both major surfaces 12 and 14 of thesubstrate. As shown in FIG. 1E, a paste composition 90 as providedherein is screen printed atop insulating layer 30 of the first majorsurface 12 and then dried. For a photovoltaic cell, paste composition 90is typically applied in a predetermined pattern of conductive linesextending perpendicularly from one or more bus bars that occupy apredetermined portion of the surface. In addition, aluminum paste 60 andback-side silver paste 70 are screen printed onto the back side (thesecond major surface 14 of the substrate) and successively dried. Thescreen printing operations may be carried out in any order. For the sakeof production efficiency, all these pastes are typically processed byco-firing them, typically at a temperature in the range of about 700° C.to about 975° C. for a period of from several seconds to several tens ofminutes in air or an oxygen-containing atmosphere. An infrared-heatedbelt furnace is conveniently used for high throughput.

As shown in FIG. 1F, the firing is carried out under conditions of timeand temperature exposure that are sufficient to cause the depicted pastecomposition 90 on the front side to sinter and penetrate through theinsulating layer 30, thereby achieving electrical contact with then-type diffusion layer 20, a condition known as “fire through.” Thisfired-through state, i.e., the extent to which the paste reacts with andpasses through the insulating layer 30, depends on the composition,quality, and thickness of the insulating layer 30, the composition ofthe paste, and on the firing conditions. A high-quality fired-throughstate is believed to be an important factor in obtaining high conversionefficiency in a photovoltaic cell. Firing thus converts paste 90 intoelectrode 91, as shown in FIG. 1F.

The firing further causes aluminum to diffuse from the back-sidealuminum paste 60 into the silicon substrate, thereby forming a p+ layer40, containing a high concentration of aluminum dopant. This layer isgenerally called the back surface field (BSF) layer, and helps toimprove the energy conversion efficiency of the solar cell. Firingconverts the dried aluminum paste 60 to an aluminum back electrode 61.The back-side silver paste 70 is fired at the same time, becoming asilver or silver/aluminum back electrode 71. It is believed that duringfiring, the boundary between the back-side aluminum and the back-sidesilver assumes the state of an alloy, thereby achieving electricalconnection. Most areas of the back electrode are occupied by thealuminum electrode, owing in part to the need to form a p+ layer 40.Since there is no need for incoming light to penetrate the back side,substantially the entire surface may be covered. At the same time,because soldering to an aluminum electrode is unfeasible, silver orsilver/aluminum back electrode 71 is formed on the back side as anelectrode to permit soldered attachment of interconnecting copperribbons or the like. Although silver paste 70 is depicted as coveringthe same area as aluminum paste 60, it is sufficient for electrode 71 tocover a limited area that still accommodates this solder attachment.

A semiconductor device fabricated as described above may be incorporatedinto a photovoltaic cell. In another embodiment, a photovoltaic cellarray includes a plurality of the aforementioned semiconductor devicesas described. The devices of the array may be made using a processdescribed herein.

It will be apparent that similar processes can be used to fabricateconductive structures in photovoltaic cells having other architecturesor other electrical, electronic, and semiconductor devices, all of whichare contemplated within the scope of the present disclosure.

EXAMPLES

The operation and effects of certain embodiments of the presentinvention may be more fully appreciated from a series of examples(Examples 1-11) described below, and comparison of those examples withComparative Examples 1-3. The embodiments on which these examples arebased are representative only, and the selection of those embodiments toillustrate aspects of the invention does not indicate that materials,components, reactants, conditions, techniques and/or configurations notdescribed in the examples are not suitable for use herein, or thatsubject matter not described in the examples is excluded from the scopeof the appended claims and equivalents thereof.

Ingredients

Ingredients useful in preparing the present paste composition includethe following. Unless otherwise stated, these ingredients are used inpreparing the Examples below.

Glass Frit:

Glass frits A and B are Pb—Te—O containing glasses having a d₅₀ value of0.5-0.7 μm.

Silver Powder

Ag powder used in the exemplary paste compositions below is finelydivided and has a predominantly spherical shape, with a particle sizedistribution having a d₅₀ value of about 1.8 to 2 μm (as measured in anisopropyl alcohol dispersion using a Horiba LA-910 analyzer).

Organopolysiloxane:

DMS-T46: polydimethylsiloxane, trimethylsiloxy terminated (Viscosity˜60,000 cSt, Gelest, Inc., Morrisville, Pa.)

Degradation Agent:

TBAF hydrate: Tetrabutylammonium fluoride hydrate (Sigma-Aldrich Co.LLC), assumed for calculations to contain 3 H₂O per (C₄H₉)₄N⁺F⁻ formulaunit.

Other Polymers:

Ethocel® STD ₄ and STD 10 are ethylcellulose-based polymers (DowChemical Company, Midland, Mich.), said by manufacturer to have ethoxylcontent of 58.0 to 49.5% and to act as rheology modifiers and binders.

Solvents:

TEX: Texanol™ ester alcohol solvent (2,2,4-trimethyl-1,3-pentadiolmonoisobutyrate) (Eastman Chemical Co., Kingsport, Tenn.)

BCA: Butyl CARBITOL™ solvent (diethylene glycol n-butyl ether acetate)(Dow Chemical Company, Midland, Mich.)

DBC: diethylene glycol dibutyl ether solvent (Aldrich)

Other Organics:

Thixatrol® PLUS amide thixotrope rheology modifier (ElementisSpecialties, Inc., Hightstown, N.J.)

Duomeen® TDO surfactant (Akzo Nobel Surface Chemistry, LLC, Chicago,Ill.) Brij™ L4: Surfactant (Croda, Inc., New Castle, Del.), polyethyleneglycol dodecyl ether, said to be a non-ionic surfactant having Mn ˜362Da.

Examples EX-1 to EX-9 Comparative Examples CE-1 to CE-3 Preparation ofConductive Paste Compositions Containing Organopolysiloxane andFluorine-Containing Degradation Agent

Unless otherwise specified, the conductive paste compositions of thepaste compositions herein, including those of Examples EX-1 to EX-9 andComparative Examples CE-1 to CE-3, are prepared in the following generalmanner.

The amounts (g) of PDMS and TBAF set forth in Table I are weighed, thenmixed to form a PDMS/TBAF mixture. The ethylcellulose polymer ispre-dispersed in BCA solvent by heating to a slightly elevatedtemperature with stirring and then cooling to room temperature. Theamounts (g) of the ethylcellulose polymer dispersion and the remainingthixotrope, surfactant, and solvents indicated for each example in TableI are weighed, then mixed with the PDMS/TBAF mixture to form a vehicle.The inorganic solids, i.e. glass frit, silver powder, and frit additive(if any) in the indicated amounts, are added and further mixed to form apaste composition. The glass frits used are Pb—Te—O based frits, butother leaded and lead-free frits might also be used. Since the silverpowder is the major part of the solids of the paste composition, it isordinarily added incrementally, with mixing after each addition toensure better wetting. Each of the foregoing mixing steps might becarried out in a planetary, centrifugal mixer. For example, a Thinky®mixer (available from Thinky® USA, Inc., Laguna Hills, Calif.) operatedat 2000 rpm for 30 s would be suitable. After the final addition, thepaste is cooled and the viscosity is adjusted to between about 80 and120 Pa-s by mixing in a small amount of added solvent. Viscosity valuesmay be obtained using a Brookfield viscometer (Brookfield Inc.,Middleboro, Mass.) with a #14 spindle and a #6 cup, with measurementafter 3 min of rotation at 50 rpm. The paste composition is thenrepeatedly passed through a three-roll mill with a 25 μm gap atpressures that are progressively increased from 0 to 400 psi (˜2.8 MPa).A suitable mill is available from Charles Ross and Son, Hauppauge, N.Y.Table I also lists a value for formulated solids, which may becalculated from the aggregate of the silver powder and glass frit ormeasured by ashing the formulated paste composition.

The degree of dispersion of each paste composition may be measured usingcommercial fineness of grind (FOG) gages (e.g., gages available fromPrecision Gage and Tool, Dayton, Ohio) in accordance with ASTM StandardTest Method D 1210-05, which is promulgated by ASTM International, WestConshohocken, Pa., and is incorporated herein by reference. Theresulting data are ordinarily expressed as FOG values represented asX/Y, meaning that the size of the largest particle detected is X μm andthe median size is Y μm. In an embodiment, the FOG values of the presentpaste compositions are typically 20/10 or less, which typically has beenfound to be sufficient for good printability.

Each processed paste composition is allowed to sit for at least 16 hoursafter roll milling, and then its viscosity is adjusted, if needed, to 80to 120 Pa-s with additional TEXANOL™ solvent to render it suitable forscreen printing. Ordinarily, the paste composition is again adjustedprior to printing by adding a small amount of solvent as required toobtain a viscosity suitable for screen printing fine lines. A finalviscosity of about 80 to 120 Pa·s (measured at 50 rpm/3 min) istypically found to yield good screen printing results, but somevariation, for example ±30 Pa·s or more, would be acceptable, dependingon the precise printing apparatus and parameters. The foregoing processis determined to produce paste composition material that is sufficientlyhomogenous to achieve reproducible solar cell performance.

TABLE I Paste Composition Formulations Ingredient CE-1 CE-2 CE-3 EX-1EX-2 EX-3 10% Ethocel ® 1.35 1.35 1.35 1.35 1.35 1.35 STD4 in BCA 10%Ethocel ® 1.35 1.35 1.35 1.35 1.35 1.35 STD10 in BCA PDMS-T46 0.70 0.770.45 0.78 0.77 0.75 TBAF hydrate 0 0 0 0.001 0.01 0.027 F/Si (calc.) 0 00 0.00023 0.0023 0.00833 Duomeen ® 0.15 0.15 0.15 0.15 0.15 0.15 TDOSurfactant Brij ™ L4 0.15 0.15 0.15 0.15 0.15 0.15 Thixatrol ® Plus 0.400.40 0.40 0.40 0.40 0.40 BCA 1.62 1.62 1.62 1.62 1.62 1.62 TEX 2.90 2.622.89 2.56 2.56 2.56 DBC 0.7 0.6 0.6 0.6 0.6 0.6 Glass Frit A 1.8 1.8 1.81.8 1.8 Glass Frit B 2.0 Silver Powder 89.3 89.24 89.24 89.24 89.2489.24 A Formulated 90.82 91.0 91.0 91.1 91.0 91.0 Solids (%) Viscosity(Pa · s, (not 95 82 100 86 79 @ 50 rpm/3 measured) min) Ingredient EX-4EX-5 EX-6 EX-7 EX-8 EX-9 10% Ethocel ® 1.35 1.35 1.35 1.35 1.35 1.35STD4 in BCA 10% Ethocel ® 1.35 1.35 1.35 1.35 1.35 1.35 STD10 in BCAPDMS-T46 0.72 0.78 0.78 0.78 0.70 0.58 TBAF hydrate 0.057 0.138 0.260.78 0.03 0.046 F/Si (calc.) 0.018 0.04 0.076 0.229 0.00782 0.018Duomeen ® 0.15 0.15 0.15 0.15 0.15 0.15 TDO Surfactant Brij ™ L4 0.150.15 0.15 0.15 0.15 0.15 Thixatrol ® Plus 0.40 0.40 0.40 0.40 0.40 0.40BCA 1.62 1.62 1.62 1.62 1.62 1.62 TEX 2.56 2.42 2.30 1.78 2.60 2.71 DBC0.6 0.6 0.6 0.6 0.7 0.6 Glass Frit A 1.8 1.8 1.8 1.8 1.8 Glass Frit B2.0 Silver Powder 89.24 89.24 89.24 89.24 89.3 89.24 A Formulated 91.091.0 91.0 91.0 90.97 91.0 Solids (%) Viscosity (Pa · s, 80 84 61 20 (not66 @ 50 rpm/3 measured) min)

Example 10 Line Dimension Characterization

The paste compositions of Examples EX-4 and Comparative Examples CE-2are screen printed on the front major surface of six inch, pseudo-squaremonocrystalline p-type silicon wafers to form solar cell precursorshaving a front-side conductive structure. The as-received wafers have afront-side n-type emitter layer and an antireflective layer. The pastecompositions differ only in the replacement of about 7.3 wt. % of thePDMS with TBAF hydrate in EX-4.

The conductive structure is formed by screen printing the requisitepaste compositions using a screen such as a Murakami 325.16 screen. (The“325.16” nomenclature indicates that the screen mesh has 325 openingsper lineal inch and the screen wire diameter is 16 μm.) The Murakami325.16 screen to be used has a 20 μm mesh thickness and provides a 15 μmemulsion thickness in a comb-like arrangement of 105 finger lines (38 μmwide) that extend from four wider bus bars. The back-side electrodes areformed by screen printing Monocrystal PASE 1206 aluminum-basedmetallization paste (available commercially from Monocrystal, Stavropol,Russia) to produce a full-plane Al—Si eutectic back contact upon firing.

The printed paste composition is rapidly dried by passing the as-printedwafers through a multizone belt furnace having a peak temperature setpoint of 350° C. During this operation, the temperature attained by thewafer is sufficient to remove at least most of the volatile componentsof the paste. After drying, the wafers are fired by passing them througha multizone belt furnace having a peak temperature set point of 885° C.to 930° C. in the hottest zone. It is understood that the peaktemperature experienced by each cell during passage through the firingfurnace in such a process may be about 140 to 150° C. lower than thesetpoint temperature in the hottest zone. After the heating step iscompleted, the organic constituents of the paste composition will havebeen substantially pyrolized, or otherwise removed, and the silverpowder sintered and adhered to the underlying silicon substrate, therebyproducing a finished conductive structure.

Line dimensions in the finger portion of the conductive structure aredetermined with a LaserTec H1200 Confocal microscope. A step and repeatprogram is used to obtain 30 individual measurements of printed fingerdimensions across the area of the wafers. An overall average iscalculated from these measurements to obtain an average line dimensionfor each particular test condition. Line dimensions of the fingers areobtained on as-printed wafers, after the paste drying step, and afterthe firing step. Line height is measured from the surface of the waferto the peak height of the line at the measurement point. An averageheight is then determined from the individual measurements. The linedimensions as thus measured are set forth in Table II for the wafers ofExample EX-4 and Comparative Example CE-2.

TABLE II Line Dimension Characterization Property CE-2 EX-4 PasteViscosity 93 87 (50 rpm/3 min, Pa · s) Fired peak height (μm) 19.4419.25 Fired width (μm) 41.27 41.25 Aspect ratio 0.47 0.47

The line dimensions and aspect ratios of the lines after firing aresimilar even though viscosity of the paste in Example EX-4 is lower thanthe paste in Comparative Example CE-2 and the compositions differotherwise only in the incorporation of TBAF in EX-4.

Example 11 Solar Cell Electrical Characterization

The electrical performance of solar cells employing front-sideelectrodes fabricated in the manner described in Example 10 ischaracterized. Measurements of light conversion efficiencies areobtained using a suitable test apparatus, such as a Berger PhotovoltaicCell Tester (Lichttechink GmbH & Co. KG). The Xe Arc lamp in the testersimulates sunlight with a known intensity of 1 sun and irradiates thefront surface of the cell. The tester uses a four-contact method tomeasure current (I) and voltage (V) at approximately 400 load resistancesettings to determine the cell's I-V curve. Light energy conversionefficiency (Eff), fill factor (FF), and series resistance (R_(s)) areobtained from the I-V curve for each cell.

For each of the Example 4 and Comparative Example 2 paste compositions,a set of 15-20 test cells is printed using a Murakami 325.16 screen thatincludes 105 fingers that are 38 μm wide. The cells are fired at aconventional peak set point temperature. The experiment is repeated toensure that reproducible results are obtained. Table III provideselectrical data for each set averaged over the test sample.

TABLE III Solar Cell Electrical Properties Property CE-2 EX-4 Efficiency(%) 19.61 19.73 Jsc (mA/cm2) 37.84 37.91 Fill Factor (%) 80.67 80.79 Voc(V) 0.6425 0.6442 Rshunt (Ohm) 59 147

The data of Table III reveal that cells having electrodes prepared withthe Example EX-4 paste show desirable increases in short circuit current(Jsc), fill factor, open circuit voltage (Voc), Rshunt, and efficiencyover those exhibited by cells having electrodes printed with ComparativeExample CE-2 paste. The increase in Rshunt in the TBAF-containingExample 4 paste composition is especially beneficial and likelyindicative of less emitter damage during firing.

Example 12 Solar Cell Electrical Characterization

The experiments of Example 11 are extended by printing additional cellswith the paste compositions of Examples 4 and 9 and Comparative Example3. The cells are printed, fired, and tested as in Example 11, and againshow desirable increases in Jsc, fill factor, Voc, and efficiency.Rshunt again shows much higher values in cells made with theTBAF-containing paste compositions.

TABLE IV Solar Cell Electrical Properties Property CE-3 EX-4 EX-9Efficiency (%) 19.57 19.67 19.62 Jsc (mA/cm2) 38.49 38.57 38.50 FillFactor (%) 79.29 79.43 79.37 Voc (V) 0.6411 0.6421 0.6422 Rshunt (Ohm)117 238 182

Having thus described the invention in rather full detail, it will beunderstood that this detail need not be strictly adhered to but thatfurther changes and modifications may suggest themselves to one skilledin the art, all falling within the scope of the invention as defined bythe subjoined claims.

For example, a skilled person would recognize that the choice of rawmaterials could include impurities or other like substances, whether ornot added intentionally, that may be incorporated into the oxidecomposition or other paste constituents during processing. For example,incidental impurities may be present in the range of hundreds tothousands of parts per million. Impurities commonly occurring inindustrial materials used herein are known to one of ordinary skill.

The presence of the impurities or other such substances would notsubstantially alter the chemical and rheological properties of the oxidecomponent, the fusible materials therein, paste compositions made withthe oxide, or the electrical properties of a fired device manufacturedusing the paste composition. For example, a solar cell employing aconductive structure made using the present paste composition may havethe efficiency and other electrical properties described herein, even ifthe paste composition includes impurities.

Where a range of numerical values is recited or established herein, therange includes the endpoints thereof and all the individual integers andfractions within the range, and also includes each of the narrowerranges therein formed by all the various possible combinations of thoseendpoints and internal integers and fractions to form subgroups of thelarger group of values within the stated range to the same extent as ifeach of those narrower ranges was explicitly recited. Where a range ofnumerical values is stated herein as being greater than a stated value,the range is nevertheless finite and is bounded on its upper end by avalue that is operable within the context of the invention as describedherein. Where a range of numerical values is stated herein as being lessthan a stated value, the range is nevertheless bounded on its lower endby a non-zero value.

In this specification, unless explicitly stated otherwise or indicatedto the contrary by the context of usage, where an embodiment of thesubject matter hereof is stated or described as comprising, including,containing, having, being composed of, or being constituted by or ofcertain features or elements, one or more features or elements inaddition to those explicitly stated or described may be present in theembodiment. An alternative embodiment of the subject matter hereof,however, may be stated or described as consisting essentially of certainfeatures or elements, in which embodiment features or elements thatwould materially alter the principle of operation or the distinguishingcharacteristics of the embodiment are not present therein. A furtheralternative embodiment of the subject matter hereof may be stated ordescribed as consisting of certain features or elements, in whichembodiment, or in insubstantial variations thereof, only the features orelements specifically stated or described are present. Additionally, theterm “comprising” is intended to include examples encompassed by theterms “consisting essentially of” and “consisting of.” Similarly, theterm “consisting essentially of” is intended to include examplesencompassed by the term “consisting of.”

When an amount, concentration, or other value or parameter is given aseither a range, preferred range, or a list of upper preferable valuesand lower preferable values, this is to be understood as specificallydisclosing all ranges formed from any pair of any upper range limit orpreferred value and any lower range limit or preferred value, regardlessof whether ranges are separately disclosed. Where a range of numericalvalues is recited herein, unless otherwise stated, the range is intendedto include the endpoints thereof, and all integers and fractions withinthe range. It is not intended that the scope of the invention be limitedto the specific values recited when defining a range.

In this specification, unless explicitly stated otherwise or indicatedto the contrary by the context of usage, amounts, sizes, ranges,formulations, parameters, and other quantities and characteristicsrecited herein, particularly when modified by the term “about,” may butneed not be exact, and may also be approximate and/or larger or smaller(as desired) than stated, reflecting tolerances, conversion factors,rounding off, measurement error, and the like, as well as the inclusionwithin a stated value of those values outside it that have, within thecontext of this invention, functional and/or operable equivalence to thestated value.

What is claimed is:
 1. A paste composition, comprising: an inorganicsolids portion that comprises: (a) a source of electrically conductivemetal, and (b) an inorganic oxide-based component, and a vehicle inwhich the constituents of the inorganic solids portion are dispersed,the vehicle comprising: (c) an organopolysiloxane; (d) afluorine-containing degradation agent; and (e) a solvent.
 2. The pastecomposition of claim 1, wherein the fluorine-containing degradationagent comprises a substance having the formula [M^(k+)][X⁻]_(k), whereinM^(k+) is a cation with positive charge k and X⁻ is F⁻ or (HF₂)⁻.
 3. Thepaste composition of claim 2, wherein the fluorine-containingdegradation agent comprises a substance having the formula M⁺X⁻, whereinM⁺ is a monovalent cation and X⁻ is F⁻ or (HF₂)⁻.
 4. The pastecomposition of claim 3, wherein M⁺ has the formula R¹R²R³R⁴N^(⊕),wherein N is nitrogen and each of R¹ to R⁴ is independently H or anyalkyl or aryl group having 1 to 15 carbons.
 5. The paste composition ofclaim 4, wherein the fluorine-containing degradation agent comprisestetrabutylammonium fluoride.
 6. The paste composition of claim 1,wherein the organopolysiloxane comprises a polymer having a structurerepresented by Formula (II),

in which each of the R¹, R², and R³ groups is independently selectedfrom C1-C8 alkyl, C2-C8 alkenyl, C2-C8 alkynyl, or C6-C10 aryl groups,with any of the R¹, R², and R³ groups optionally comprising one or moresubstituents selected from alkoxy, hydroxy, carbonyl, carboxyl, amino,epoxy, methacrylic, glycidoxy, ureido, sulfide, methacryloxy,sulfhydryl, and halogen groups; and n is an integer ranging from 50 to100,000.
 7. The paste composition of claim 6, wherein theorganopolysiloxane comprises polydimethylsiloxane.
 8. The pastecomposition of claim 1, wherein the organopolysiloxane comprises aco-polymer having a structure represented by Formula (III),

in which each of the R¹, R², R³, and R⁴ groups is independently selectedfrom C1-C8 alkyl, C2-C8 alkenyl, C2-C8 alkynyl, or C6-C10 aryl groups,with any of the R¹, R², R³, and R⁴ groups optionally comprising one ormore substituents selected from alkoxy, hydroxy, carbonyl, carboxyl,amino, epoxy, methacrylic, glycidoxy, ureido, sulfide, methacryloxy,sulfhydryl, and halogen groups, and with the proviso that the R¹ and R⁴groups are different; and m and n are integers, each of which may rangeindependently from 50 to 100,000, and with the proviso that the R¹ andR⁴ groups are different.
 9. The paste composition of claim 1, whereinthe organopolysiloxane has a number average molecular weight Mn rangingfrom 5 to 1,000 kDa.
 10. The paste composition of claim 1, wherein theamount of organopolysiloxane ranges from a lower limit of 0.05%, 0.1%,0.2%, or 0.3% to an upper limit of 0.75%, 1%, or 2% by weight of thepaste composition.
 11. The paste composition of claim 1, wherein a ratioof the number of F atoms present in the degradation agent to the numberof Si atoms present in the organopolysiloxane ranges from a lower limitof 0.0002, 0.001, 0.002, 0.005, or 0.01 to an upper limit of 0.05, 0.1,or 0.2.
 12. The paste composition of claim 1, wherein the electricallyconductive metal comprises silver.
 13. The paste composition of claim 1,wherein the source of electrically conductive metal comprises 85 to99.75% by weight of the solids.
 14. The paste composition of claim 1,wherein the oxide-based component comprises 0.25 to 15% by weight of thesolids.
 15. The paste composition of claim 1, wherein the inorganicoxide-based component comprises a lead tellurium oxide glass frit thatoptionally comprises at least one of 0.25 to 5 wt. % B₂O₃ or 0.1 to 5wt. % Li₂O.
 16. A process comprising: (a) providing a semiconductorsubstrate having opposing first and second major surfaces and comprisingan insulating layer situated on the first major surface of thesemiconductor substrate; (b) applying a paste composition as recited byany previous claim onto at least a portion of the first major surface,and (c) firing the semiconductor substrate and the paste compositionunder conditions sufficient for the paste composition to penetrate theinsulating layer and form an electrode in electrical contact with thesemiconductor substrate.
 17. An article made by the process of claim 16.18. The article of claim 17, wherein the article comprises aphotovoltaic cell.
 19. A photovoltaic cell precursor, comprising: (a) asemiconductor substrate having opposing first and second major surfaces;and (b) a paste composition as recited by claim 1, the paste compositionbeing applied onto a preselected portion of the first major surface andconfigured to be formed by a firing operation into an electricallyconductive structure comprising an electrode in electrical contact withthe substrate.